In the fuel vapor treatment system, fuel vapor generated in a fuel tank is temporarily adsorbed by adsorbent in a canister. Desorbed fuel vapor is mixed with air and is purged into the internal combustion engine, so that the fuel vapor is combusted with injected fuel in a combustion chamber of the internal combustion engine. In a system shown in JP-2006-312925A (US2006/0225713A1), fuel vapor concentration of the mixture gas is detected to correctly control the quantity of the purge gas.
Specifically, a purge passage is connected to a detection passage. The mixture gas of the fuel vapor desorbed from the canister and air is introduced into the detection passage, so that the fuel vapor concentration of the mixture gas is detected. Since the fuel vapor concentration is detected before purging and its detected value is reflected to the purge control from its starting time, a disturbance of the air-fuel ratio is restricted.
In a system shown in JP-2006-312925A, the detection of fuel vapor concentration is periodically performed. The detection interval of the fuel vapor concentration is set to a constant value. In a case that the detection interval is excessively long, the actual fuel concentration may deviate from the detected concentration, which may cause a disturbance of air-fuel ratio. In a case that the detection interval is excessively short, an operation frequency of a pump that generates gas flow to introduce the mixture gas into the detection passage may increase. It may cause a deterioration of the parts and its endurance.